不同氮浓度和形态比例对水曲柳幼苗叶绿素合成、光合作用以及生物量分配的影响

摘要

水曲柳(Fraxinus mandshurica)是我国东北林区重要的工业用材树种,在东北林区广泛种植,因而其培育近来日益得到高度重视.在水曲柳的种植区域内,尽管林地内凋落物丰富,但该地区气温低,冬季长,氮素矿化速度低,氮素供给显得不足.本研究采用沙培的方式,在为幼苗提供完全平衡营养液30 d后,对幼苗进行4种不同的氮素浓度(1、4、8、16 mmol*L-1)或5种氮素形态(NO-3-N和NH+4-N)比例(0、25%、50%、75%、100%)的处理.氮素浓度处理使用NH4NO3,氮素形态处理中NO-3-N使用KNO3,NH+4-N使用NH4Cl.每一种处理方式重复15次.随后对不同的氮浓度和形态比例对水曲柳幼苗的叶绿素合成、光合作用、生物量的累积以及生物量在地下部分和地上部分的分配的影响进行了研究,旨在进一步了解氮素浓度和氮素形态的比例对水曲柳幼苗生理生态学的影响.在氮素浓度处理中,5-氨基-酮戊酸(5-aminolevulinic acid, ALA)的合成速率随氮素浓度呈现不规则变化,在1 mmol*L-1处达到最大(0.72 μmol*g-1 FW*h-1),在4 mmol*L-1处最小.当氮素形态比例从0增加到50%时, ALA合成速率逐渐增加,而当氮素形态比例再增加时(75%～100%),ALA合成速率反而减少.氮素浓度对胆色原素(Porphobilinogen,PBG)合成酶活性没有显著影响,而当氮素形态比例从0增加到75%时,PBG合成酶活性逐渐增加,只对幼苗供给NO-3-N时,PBG合成酶活性又下降.氮素浓度从1 mmol*L-1增加到8 mmol*L-1时,叶绿素a、叶绿素b以及总叶绿素含量增加,但PBG合成酶活性对这3个指标没有影响.当氮素形态比例从0增加到75%时,总叶绿素含量逐渐增加,而其它两个指标则呈现不规则变化,并且总叶绿素含量变化与PBG合成酶活性没有显著关系.当氮素浓度从1增加到8 mmol*L-1时,叶片总氮和可溶性蛋白增加,而过多的氮素供给(16 mmol*L-1)并不能使叶片合成和累积更多的氮和可溶性蛋白.当氮素形态比例从0增加到75%时,叶片总氮和可溶性蛋白含量增加.尽管水曲柳幼苗偏好NO-3-N,但完全供给NO-3-N时,叶片总氮和可溶性蛋白含量下降.当氮素浓度从1增加到8 mmol*L-1时,净光合速率增加;过多供给氮素(16 mmol*L-1)净光合速率反而下降.当氮素形态比例从0增加到75%时,净光合速率增加,而完全供给NO-3-N时,光合速率下降.相似地,随外源氮供给的浓度从1增加到8 mmol*L-1,总生物量也增加;与此同时,总生物量在地下部分的分配减少,在地上部分的分配增加.随NO-3-N比例从0增加到75%, 总生物量也增加.在氮素形态处理中,生物量在地下部分的分配随NO-3-N比例增加而增加.无论是氮浓度处理还是氮素形态处理,生物量的累积与光合作用强度有密切的关系,且这种关系与氮素在叶片中的分配有直接的联系.根据实验结果,在水曲柳幼苗的培育过程中,氮素浓度为8 mmol*L-1为益,氮素形态比例为75%∶25%为益.%Fraxinus mandshurica, native to northeastern China, is an important timber tree. The tree often suffers from nitrogen (N) deficiency because long winter results in slow decomposition rate of litter. In the present study, effects of different concentrations and form ratios of nitrogen on chlorophyll biosynthesis, total leaf N, soluble protein, and photosynthesis were studied. During the N concentration treatment, biosynthesis rate of 5-aminolevulinic acid (ALA) and activity of porphobilinogen (PBG) synthase showed irregular changes, but total chlorophyll content increased when N concentrations varied from 1 to 8 mmol*L-1, and activity of PBG synthase did not change significantly. Total leaf N, soluble leaf proteins, and photosynthesis rate also increased when N concentrations increased from 1 to 8 mmol*L-1. These results suggested that N concentrations did not affect biosynthesis rates of ALA and activities of PBG synthase and 8 mmol*L-1 of N most increased total leaf N, soluble proteins in leaves, and photosynthesis rate. During the N form treatment, biosynthesis of ALA increased when the ratio of NO-3-N increased from 0 to 50%, but activity of PBG synthase, total chlorophyll concentrations, total leaf N and soluble leaf proteins increased when the ratio of NO-3-N increased from 0 to 75%. Photosynthesis rate also increased when the ratio of NO-3-N increased from 0 to 75%. During the two types of treatments, there were significant relationships between total leaf N, soluble protein concentrations and net photosynthesis rates. These results suggested that F. mandshurica seedlings showed a preference for NO-3-N, but the ratio of 75% NO-3-N to 25% NH+4-N promoted an increase in chlorophyll biosynthesis, accumulation of N and soluble proteins in leaves, and net photosynthesis rate. Total biomass increased as exogenous nitrogen concentrations increased from 1 to 8 mmol*L-1. Biomass partitioning to roots decreased, and biomass partitioning to shoots increased. As ratios of NO-3-N to NH+4-N increased from 0 to 75%, total biomass increased, but biomass partitioning to roots increased as ratios of NO-3-N to NH+4-N increased from 0 to 100%. When both nitrogen concentration treatment and form ratio treatment were carried out, biomass accumulation was closely related to photosynthesis rate, and the relationship between biomass accumulation and photosynthesis was related to nitrogen partitioning in leaves. It is proposed that 8 mmol*L-1 and 75% NO-3-N∶25%NH+4-N should be used in practical cultivation of F. mandshurica seedlings.